The storage and transport of corrosive materials (sometimes referred to as “aggressive” materials) can lead to a variety of problems. The magnitude of these problems increases with the corrosiveness of the materials. Some aggressive substances produce a considerable amount of corrosion damage, requiring actual repair of the transport or storage container or conduit used to transport or store the substance.
Fertilizer solutions, such as nitrogen-based solutions, are very corrosive. Example nitrogen-based solutions include ammonia solutions, Ammonium Nitrate (AN), and urea ammonium nitrate (UAN). AN and UAN are commercially popular and are widely used in agricultural applications. But due to their extremely high corrosiveness, AN and UAN solutions are problematic to store and transport. Similar corrosiveness problems exist for storing and transporting a variety of materials.
The popularity of UAN is due in part to its relative ease of handling. For example, UAN has an extremely low critical relative humidity (the atmospheric humidity at which the solid product absorbs water from the atmosphere) of only 18% at 30° C. Thus, UAN is used as a liquid fertilizer, and does not need to be kept under pressure. As a liquid, it can be injected beneath the soil surface, dribbled onto the soil surface, added to irrigation water, or sprayed onto plant leaves and/or the soil surface at controlled application rates. In addition, it can be easily mixed with herbicides, pesticides, micronutrients, or other liquid fertilizers, allowing for one-pass application. This substantially reduces the time and cost required to apply these components to a crop.
AN and UAN deliver a high concentration of both fast- and slow-release nitrogen nutrients. The nitrate (NO3−) portion of these fertilizers (typically accounting for about 25% of the total nitrogen content in UAN) is immediately available for plant uptake. The ammonium (NH4+) portion (typically about another 25% of the total nitrogen content in UAN) may be assimilated directly by some plants, but is also rapidly oxidized by soil bacteria to form nitrate. Soil enzymes hydrolyze the remaining urea portion (typically about 50% of the total nitrogen content in UAN) to form ammonium, which is subsequently transformed into more nitrate in the soil. Thus, the nitrogen nutrients can either be immediately absorbed by the plants, or absorbed more gradually through the soil.
AN and UAN fertilizers are also simple to produce. To produce UAN, for example, a heated solution containing dissolved urea is mixed with a heated solution of ammonium nitrate to create a clear solution of urea ammonium nitrate. UAN can either be made in a batch or continuous process. No emissions or waste products are produced during mixing.
Because AN and UAN are concentrated nitrogen solutions, their solubility decreases as temperature decreases. To prevent the nitrogen components from precipitating as crystals, manufacturers dilute the solutions in geographic regions that may experience colder seasonal climates. UAN, for example, is typically manufactured with about 20 wt % water, and is generally further diluted for field applications to about 28 wt % water. An may be diluted with, for example, 30 wt % to 50 wt % water.
AN solutions generally can include a total nitrogen content in the range of from 10% to 50%, and can include from 50 wt % to 100 wt % of ammonium nitrate, and from 0 wt % to 50 wt % of water. More typically, AN solutions can include from 15% to 40% nitrogen, or from 20% to 35% nitrogen; from 40 wt % to 80 wt %, or from 50 wt % to 70 wt % of ammonium nitrate; and from 20 wt % to 60 wt %, or from 30 wt % to 50 wt % of water. For example, AN 20 (also known as AN 20-0-0) typically has about 50 wt % to 70 wt % ammonium nitrate, and about 30 wt % to 50 wt % water. Its total nitrogen content is about 20%. By comparison, the total nitrogen content of AN 34 is about 34%. Other concentrations may also be used.
UAN solutions generally can include a total nitrogen content in the range of from 10% to 50%, and can include from 20 wt % to 80 wt % of each of ammonium nitrate and urea, and from 0 wt % to 50 wt % of water. More typically, UAN solutions can include from 15% to 40% nitrogen, or from 20% to 35% nitrogen; from 25 wt % to 60 wt %, or from 30 wt % to 45 wt % of each of ammonium nitrate and urea; and from 10 wt % to 40 wt %, or from 20 wt % to 30 wt % of water. For example, UAN 32 (also known as UN32, UN-32, or UAN 32.0.0) typically has about 45 wt % ammonium nitrate, about 35 wt % urea, and about 20 wt % water. Its total nitrogen content is about 32%. UAN 28 typically has about 39 wt % ammonium nitrate, about 31 wt % urea, and about 30 wt % water, and its total nitrogen content is about 28%. Other concentrations may also be used, such as UAN 18 or UAN 30.
Despite its advantages, a major downside of both AN and UAN is that they are highly corrosive to storage tanks, pumps, pipes, rail cars, barges, tank trucks, and agricultural application equipment. UAN is particularly corrosive toward mild steel (that is, low-carbon steel having approximately 0.05%-0.25% carbon), which is commonly used in such applications. For example, UAN can lead to corrosion of up to 500 mils penetration per year (MPY) on C1010 steel. UAN solutions are acidic, with a pH of typically 6.0 or lower. The pH of UAN solutions will drift even lower as any buffering agents added to the solution are volatilized. Traditional corrosion inhibitors are ineffective at preventing corrosion in such acidic conditions unless added in large amounts. Thus, corrosion is a persistent problem in the production, storage, transportation, and application of AN and UAN solutions. If left untreated, corrosion can lead to several detrimental impacts, including unscheduled plant shutdowns, discoloration and sludge formation, damage to storage tanks and rail cars, increased maintenance and downtime costs, and increased financial and environmental claims.
Amine-based buffering agents, such as ammonia, are typically added to UAN after production to protect equipment and storage/transport vessels. Ammonia itself is not a corrosion inhibitor. However, the ammonia acts to buffer the corrosive effect of the ammonium nitrate, which is the most corrosive component of UAN. As little as 0.01 wt % (100 ppm) to as much as 0.20 wt % (2,000 ppm) excess ammonia may be added as a buffer. Without the addition of this small excess of ammonia, UAN will become extremely acidic, and conventional corrosion inhibitors cannot be applied cost effectively. However, high levels of ammonia can be a safety hazard, and some herbicides and other tank mix products may be sensitive to the presence of ammonia in the solution. Smaller levels of ammonia may not be sufficient to act as a buffer, and corrosion can easily occur once the vessel is emptied, leaving behind a thin UAN film on the vessel wall and residual UAN at the bottom of the vessel (sometimes referred to as a “heel”).
Due to this formation of a thin film, surface corrosion occurs on the vertical walls of the vessel. “Surface corrosion” is the typical form of corrosion produced by low ammonia levels and is itself not very damaging to the vessel initially. However, surface corrosion results in the generation of a large amount of corrosion sludge, which winds up on the vessel floor. The sludge can plug spray nozzles in fertilizer application equipment and irrigation booms and contribute to discoloration of the next fertilizer load.
More significantly, sludge formation leads to a corrosion mechanism known as “under-deposit corrosion.” The corrosion sludge prevents the normal flow of ions in the solution, resulting in “pockets” of corrosive ions under the sludge. The buildup of corrosion sludge can also prevent corrosion inhibitors from accessing the vessel surface under the sludge. As a result, pits as deep as a quarter inch can form in as little as 5 years.
General remedies used in the past to inhibit AN- or UAN-caused corrosion include high levels (usually hundreds or thousands of mg/kg) of phosphate or polyphosphate salts added directly to the AN or UAN solution to serve as bulk corrosion inhibitors. These remedies fell into disfavor, however, because the phosphates precipitated with other constituents, such as iron, calcium, magnesium, etc. Such precipitates led to unfavorable deposits on the bottom of vessels (as described above), as well as plugging of spray application devices.
Various types of organic film-forming corrosion inhibitors (“filmers”), such as phosphate esters and the like, have also been added to AN and UAN solutions as bulk corrosion inhibitors, but these typically suffer from several problems. Due to filmers' surfactant nature, they may contribute to undesirable foaming during loading or unloading of the fertilizer. Some anti-foam additives can become less effective with time, so the foaming problem can be addressed initially, but may often become problematic before or during application of the fertilizer solution. If the filmer is not well dispersed in the solution, it becomes less effective.
The selection of corrosion inhibitors for liquid fertilizer solutions is made more difficult by the presence of environmental considerations. Since the fertilizer solutions are applied to crops, for example, they must be free of compounds which are toxic to the crops being fertilized, and must also facilitate compliance with industrial hygiene standards for the personnel applying the fertilizer. Thus, fluoride compounds, as one example, are undesirable in UAN solutions because they are generally agrotoxins.
Other corrosion inhibitors that are sometimes used in boilers and cooling towers, such as zinc, are incompatible with the relatively more severely corrosive AN and UAN. Other well-known corrosion inhibitors, such as molybdate and tungstate, do not prevent bloom rust formation (discoloration of a surface finish indicating the early stages of rust) in storage and transport vessels. These “passivating” corrosion inhibitors function by forming insoluble complexes with Fe2+ ions as they are generated at the metal surface. These types of corrosion inhibitors are thus not effective at preventing rust/sludge build-up, particularly when large amounts of Fe2+ ions are present. These insoluble iron ions can form complexes when brought into contact with acidic AN and UAN solutions and contribute to voluminous sludge formation/deposition and under-deposit corrosion. Furthermore, their performance deteriorates as the pH of the AN or UAN decreases, which occurs as the ammonia volatizes. Molybdate-based corrosion inhibitors also have environmental concerns.
In addition to the above-described bulk corrosion inhibitors directly added to the AN or UAN solution, vessel coatings have also been developed in an attempt to prevent and inhibit corrosion. Such coatings provide a layer on the inner surface of a vessel to prevent contact of the AN or UAN with the inner surface of the vessel. However, the addition of AN or UAN leads to the rapid removal of mineral oil coatings, and rubber or epoxy liners can suffer from cracking or pinhole defects that lead to rapid pitting of any small exposed metal surface. Also, such liners are very often cost prohibitive.
There thus exists a need to provide improved corrosion resistance for storage and transport vessels that carry corrosive substances. In particular, there exists a need to inhibit corrosion on the inner surface of stationary and mobile transport vessels that hold nitrogen-based solutions and other corrosive materials, such as AN or UAN.